The response of an electronic device is often dependent on external physical parameters such as temperature, pressure, and humidity. For example, as shown in FIG. 1, the output power of a laser diode is a function of both the temperature of the diode, and the input current injected into the diode. At a given temperature, a minimum or threshold current must be supplied to the laser diode to cause it to emit a significant amount of power in the form of laser light. Above the threshold current, the power output of the laser diode is approximately proportional to its input current. This dependence of output power on input or injected current allows information to be modulated onto the power output of the laser diode by modulating the input current used to drive the laser diode.
Modulation of the output power of a laser diode is typically achieved by driving the laser diode with both a bias current 120 and a modulation current 130 as shown in FIG. 1. The combination of the bias 120 and modulation 130 currents establishes an operating range for the laser diode within which the laser light output power 140 is modulated. The operating range includes a minimum light output power level 150 and a maximum light output power level 160. When the modulation current 130 is digitally modulated between low and high current levels, laser light output power 140 is similarly modulated between low 150 and high 160 power levels. The low 150 and high 160 power levels can be used to represent the binary logic levels 0 and 1 in a digital bit stream. Thus, the laser diode can be used to generate and transmit a digital bit stream by driving it with a modulation current 130 that is driven by the same digital bit stream.
It is well known that the power output of a laser diode has a strong temperature dependence. Consequently, the operating range established for a laser diode by a given pair of bias and modulation currents changes as the ambient temperature of the diode changes. For many applications, it is important to maintain the light output power levels of a laser diode within a predetermined operating range. For example, in optical fiber communications it is important to maintain a laser diode's light output power levels within a predetermined operating range so that the system can discriminate between the low and high logic levels corresponding to the laser diode's low 150 and high 160 light output power levels.
To maintain a laser diode's established low 150 and high 160 light output power levels as the diode's temperature changes, the diode's bias and modulation currents must be temperature compensated or adjusted to correct for the dependence of the laser diode's light output power on temperature. For example, as shown in FIG. 1, as the temperature of a laser diode configured to operate at −40° C. between low 150 and high 160 light power output levels changes from −40° C. to +80° C., the diode's bias 120 and modulation 130 currents at −40° C. must be adjusted to temperature compensated bias 121 and modulation 131 currents at +80° C. in order to keep the diode's light output power within its operating range.